With the advent of the laser, many devices were soon developed for both steering and modulating information-bearing intelligence upon the near infrared and visible optical beam produced by the laser. It was quickly recognized that its spatially coherent monochromatic electromagnetic optical energy enabled its beam to carry wideband information. That is, audio, video or digital information, which conveyed intelligence, could be modulated upon and transmitted by means of the laser beam because of its spectral qualities.
The development of improved gas and solid state lasers for feeding optical-fibre systems for transmission of information has also lead to the development of free-space or atmospheric optical systems such as is disclosed in U.S. Pat. No. 4, 627,106. It is taught therein that a beam of spatially coherent, monochromatic, aperture limited, electromagnetic optical energy can be modulated, thereafter propagated through free space as mutually aligned wavefronts and subsequently detected by a receiving means for deriving and utilizing the information provided by the modulation. The document teaches the importance of maintaining the wavefront alignment in orientation throughout the system less the information becomes garbled by intermixing of the waves.
Thus, optical free-space communication channels have been established using laser sources in both the visible and infra-red wavelength bands. Their high directionality is one of most appealing characteristics of laser beams and this property is often associated with their high degree of spatial coherence and sought as such in optical free-space transmissions (see U.S. Pat. Nos.: 4,764,982; 4,928,317; and, 5,457,561).
High volume communication systems is perhaps the most rapidly developing economic sector. Advances in fiber optic communication systems have brought tremendous market opportunities. One potential bottleneck in the wide spread of fiber optic systems is the xe2x80x9clast milexe2x80x9d connection to consumers as well as specific requirements of xe2x80x9clocal area networksxe2x80x9d. For obvious economic reasons, the first choice in these situations in communication systems is the broadband wireless systems. The available options are radio, microwave, and free-space optical connections. Optical free-space communication systems have definite advantages such as: broadband; inexpensive multiplexing; no special operation licensing (unlike microwaves); ease of installation and fast deployment; highly secure (direct link); and, fully portable.
Operation at more than 2.5 Gbits/second has been demonstrated for using broadband, ultrahigh speed optical transmission over few miles. In principle, full benefits of fiber optic communications (speed, volume, etc.) can be obtained in free-space connections.
There is however one potential drawback; light beams propagation even over distances of few kilometers can be affected by adverse conditions like atmospheric turbulences, rain or aerosols. As a result, beam wondering and/or attenuation could lead to a decrease in the degree of confidence of a free-space optical communication channel. The problem is especially relevant for situations where the atmospheric conditions change rapidly, in regions where humidity is high or where substantial amount of particulates is present in the horizontal communication path (for instance, desert or coastal regions). Obviously, these issues limit the distances over which secure optical communications can be operated.
To limit such atmospheric effects, the complexity of an optical communication system has to be increased. This usually means that several optical channels are needed to be operated in parallel and the transmitted information is cross-correlated to account for different perturbations in different channels. To limit the beam distortions, one can enlarge the size of the transmitted beam but this eventually limits the range of practical applications since the major advantages of an optical system are its size, ease of installation and operation.
The first objective of the present invention is to provide stable free-space optical communications.
The second object of this invention is to provide stable free-space optical communication in adverse atmospheric conditions.
The third object of this invention is to utilize low-coherence optical beams to provide for stable free-space optical communications.
The fourth objective is to provide for a stable free-space optical communications system of enhanced stability utilizing a laser generated partially coherent optical beam propagated through turbulent atmospheres
The fifth objective is to develop a modulated laser source that will be appropriate for advanced telecommunications and optical signal processing applications.
A preferred embodiment fully describing the invention (which arose out of the discovery that partially coherent laser generated optical beams are less influenced upon propagation through turbulent free-space atmospheres than are coherent optical beams) is a communication system for transmitting intelligence from a transmitter site through non-confined free space and receiving said intelligence at a receiver site remote from and physically separate from said transmitter by non-confined free space, said system comprising: means at said transmitter site for producing a beam of spatially-coherent monochromatic, aperture limited electromagnetic optical energy; means for modulating said beam with intelligence-bearing information to develop wavefronts of mutually-aligned orientation; means for modifying said beam to partial coherence; means at said receiver site for detecting said information in said partially coherent beam; and, means for deriving said information demodulated from said detecting means.
Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment, which is illustrated, schematically in the accompanying drawings.